Recently, accompanied by the increase of environmental awareness, waste disposal problem of plastic products has drawn attention. Regarding shape retaining tools used as presentation packaging for various articles and containers such as trays for food or beverage cups, those in which various biodegradable plastic sheets are used has been developed. Among them in particular, polylactic acid has mostly drawn attention as an expectable material, because its glass transition temperature is high as approximately 60° C. despite being a biodegradable plastic, or of its transparency.
However, the polylactic acid is low in glass transition temperature by approximately 20° C. compared to materials derived from petroleum, i.e., polyethylene terephthalate, and there is a problem that heat resistance is insufficient when used in each of present applications.
As a means for solving this problem, a technique is proposed in which the heat resistance is improved by increasing crystallinity of polylactic acid resin by incorporating a nucleating agent in the polylactic acid resin.
For example, in JP-H9-278991 A and JP-H11-5849 A, a technique is described in which crystallinity is increased by heat treating a polylactic acid sheet containing a transparent nucleating agent, at the time of thermoforming or after thermoforming. However, in this technique, since the nucleating agent is contained in the whole resin, the effect of crystallization of the sheet by heat at the time of thermoforming is significant, and there was a problem in thermoformability.
Furthermore, in JP 2004-149692 A, a technique is described in which a thermoformed article which is high in folding endurance and excellent in heat resistance is obtained by incorporating a nucleating agent to a polymer component consisting of a lactic acid component and mainly an aliphatic polyester component. However, by this technique, although folding endurance was improved, thermoformability of the sheet was insufficient.
Furthermore, in JP 2005-119061 A and JP 2005-119062 A, a polylactic acid-based sheet of which crystallization rate is high and folding endurance is large is proposed, by employing a two-types-three-layers sheet constitution consisting of, as the inner layers, a lactic acid-based polyester mainly consisting of an aliphatic polyester component of polylactic acid and a lactic acid component and, as the outer layer, a polylactic acid containing a transparent nucleating agent. However, in the technique disclosed here, there was a problem that transparency was deteriorated by heat treatment at the time of thermoforming.
It could therefore be advantageous to provide a laminate sheet of polylactic acid-based resin from which a thermoformed plastic excellent in heat resistance and transparency can be obtained and which is excellent in thermoformability.